This invention relates to inductor materials having exceptional temperature stability. More particularly, it involves a nickel-zinc ferrite FM tuning core which is interchangeable with a powdered iron core, yet which has a substantially lower temperature coefficient of permeability.
In tuned circuit applications it has been commonplace to provide a core having a high magnetic permeability which can be inserted into a tuning coil to increase or decrease the inductance thereof depending upon the degree of insertion. Powdered iron cores have been commonly used in FM tuning applications fairly successfully. However, one disadvantage of the powdered iron core is that its magnetic permeability varies with temperature. This can present a problem in applications where the iron core is subjected to wide temperature fluctuations. For example, in automotive radio applications the range of temperatures to which the core can be subjected is approximately -36.degree.F to 186.degree.F.
Until now the powdered iron core, even with this limitation, has been one of the most practical materials for automotive radio applications. Hence, many FM automotive radio tuning circuits have been designed for its use therein, taking into account its various magnetic properties such as permeability, etc. I have discovered a particular nickel-zinc ferrite composition that can be used to make a tuning core that is interchangeable with a powdered iron core. The ferrite core of my invention has magnetic properties almost identical to that of a powdered iron core except that the ferrite core of my invention has a substantially lower temperature coefficient of permeability. Therefore, my ferrite core can be interchanged with a powdered iron FM tuning core without the necessity of redesigning the electrical tuning circuitry. This means that the temperature stability of an FM tuning circuit can be increased merely by substituting a ferrite core made in accordance with my invention for the powdered iron core for which the circuit has been designed. Consequently, a gain in radio performance may be had without the loss of time and expense of circuit redesign.
I recognize that ferrite bodies of various compositions have been used as inductor materials. For example, U.S. Pat. No. 3,142,645 Zerbes discloses a nickel-zinc ferrite composition having 2-31.5% ZnO, 2-29.5% NiO, and 66.5-80% Fe.sub.2 O.sub.3 with the addition of 0.1-3% CoO. U.S. Pat. No. 3,242,089 Bartow et al discloses a nickel-zinc ferrite composition of 1-40% ZnO, 1-40% NiO, and 50-89% Fe.sub.2 O.sub.3 with the addition of 0.1-3% CoO. However, none of the prior art recognizes that a very specific nickel-zinc ferrite composition, namely 4.5-5.5% ZnO, 25-30% NiO, and 65-70% Fe.sub.2 O.sub.3, will display the same magnetic characteristics of a powdered iron tuning core. This specific ferrite composition permits a radio manufacturer to merely replace the old powdered iron core with the ferrite core of this invention without having to redesign the radio tuning circuitry, and yet gain a substantial improvement of radio performance. This feature is particularly attractive to high volume manufacturers who have already spent a considerable amount of money for circuit design, and have well established production facilities which have been adapted to that circuit design which has proved highly reliable over the course of time. Conceivably, the prior art ferrite compositions can be used as an FM radio tuning core, yet in order to incorporate them into the radio considerable expense must be had to redesign the circuitry to adjust to their different magnetic properties.